European Patent No. 0666148 discloses a hammer drill having a means for detecting reaction torque acting on a tool body in a direction opposite to a direction of rotation of a hammer bit. In a power tool such as a hammer drill, when a hammer bit is unintentionally locked during hammer drill operation, reaction torque acting on the tool body increases so that the tool body may be swung. In the above-described known hammer drill, a rotation sensor is provided which monitors rotation of the tool body when the tool body rotates around a rotation axis of the hammer bit by reaction torque acting on the tool body. The rotation sensor predicts a future uncontrollability of the tool body from angles observed within a fixed period of time and interrupts torque transmission between the motor and the hammer bit.
In the construction in which the rotation sensor is used to predict a future uncontrollability of the tool body, for example, when the user performs an operation while rapidly moving the tool body on his or her own will, even if the tool body is not rendered uncontrollable, the rotation sensor may incorrectly determine the tool body to be uncontrollable and interrupt torque transmission. Specifically, in the known technique of detecting reaction torque acting on the tool body by the rotation sensor, further improvement is required in accuracy of detection.